Porphyria cutanea tarda is the most commonly occurring type of the group of enzyme deficiencies classified as porphyria. It is caused by insufficient levels of uroporphyrinogen III decarboxylase, or UROD, which encodes the fifth enzyme required in the production of heme, an important molecule in human physiology. Heme is part of the hemoglobin molecule that transports oxygen throughout the blood. Porphyria cutanea tarda is characterized by sensitivity to sunlight, causing the skin to blister when exposed to too much sun.

Though porphyria cutanea tarda occurs more often than other types of porphyria, it is very rare, affecting only one in 25,000 people. About 80% of cases are sporadic, or not inherited. In addition, the disorder is often asymptomatic.

The blisters caused by photosensitivity in patients of this disease most often affect the face, lower legs, and forearms. They heal slowly and often scar. In some patients, photosensitivity manifests not as blisters, but as hyperpigmentation, or darkening of the skin, or as hypertrichosis, abnormal hair growth, often on the face. In patients that have not inherited the disease, liver inflammation and scarring are common.

Sporadic porphyria cutanea tarda differs from the inherited disease in that the UROD gene shows no mutations. Rather, the body has an abnormally high demand for heme, as well as for the enzymes involved in heme production. Risk factors for porphyria cutanea tarda include alcoholism, excessive iron or estrogen, hepatitis C infection, cancer, and mutations of the hemochromatosis protein or HFE gene. These risk factors can also exacerbate the disease in people with an inherited UROD gene mutation.

Inherited or familial porphyria cutanea tarda is sometimes referred to as Type I, while the sporadic variety is classified as Type II. The rarest subtype, Type III, shows a familial pattern in that more than one family member is affected, but does not involve a mutation of the UROD gene. HFE gene mutations may be the genetic factor in such case.

Porphyria cutanea tarda is usually diagnosed through a urine test revealing high levels of uroporphyrinogen. If laboratory tests do not reveal any signs of the disease, but the patient is experiencing its symptoms, the diagnosis is often pseudoporphyria. This condition is not well understood, but may be caused by an allergic reaction to medication, or by excessive exposure to long wave ultraviolet (UVA) rays, as in a tanning booth.

Porphyria cutanea tarda is a chronic condition with no known cure, so treatment is aimed at alleviating the symptoms. Patients are advised to avoid alcohol, iron-rich foods, sunlight, and estrogen. Treatment may also involve treatment of hepatitis C as needed, bloodletting to decrease iron in the body, and medication with chloroquine, also used to treat and prevent malaria.

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